The present invention is directed to a fuel composition comprised of dewatered sewage sludge and a liquid hydrocarbon fuel oil.
In an effort to provide an economic and environmentally acceptable method for the disposal of sewage sludge, various municipalities have begun to investigate various means to accomplish this difficult task. Disposal of sewage sludge is rapidly becoming a world-wide problem. Many large municipalities on ocean coastlines dump sewage sludge produced by them into the oceans with the consequence of ultimately endangering ocean aquatic life. Inland municipalities frequently employ the sludge as a source of plant nutrients.
However, the supply of sewage sludge is rapidly exceeding demand. As a result, the sewage sludge is frequently disposed of by means of landfill. Others employ expensive and involved incineration methods to dispose of their sludge whereby the sludge is first dried to form a combustible solid. Because of the rapid increase in the world population over the last 100 years and the shift from an agrarian-based to urban-based culture, the use and/or disposal of increasing quantities of sewage sludge is rapidly becoming a problem of alarming proportion. Furthermore, toxic and hazardous wastes comprise an important component of sludge, adding to the complexity and safety of current sludge disposal techniques since such components are highly leachable. Therefore, it would be extremely advantageous to provide an economic and environmentally acceptable alternative means of disposal of such sewage sludge, thereby avoiding the problems associated with landfills and ocean dumping.
Two incineration methods are commonly employed for the incineration of sewage sludge; i.e., fluidized beds and multiple hearth furnaces. Both methods were developed to provide on-site sewage sludge capability at sewage treatment plants. Each method employs efficient heat transfer mechanisms to minimize fuel consumption which limits the exposure of the sludge to high temperatures within the incinerator. Afterburners are accordingly frequently employed to minimize the discharge of offensive odors from such incinerators. Maximum temperatures on the order of about 815.degree. C. are attained in fluid bed incinerators. Multiple hearth furnaces expose freshly fed sludge to cool combustion gases as low as 65.degree. to 160.degree. C. prior to entering the final combustion zone at 750.degree. to 950.degree. C.
Various methods have been discussed in the patent literature for the disposal of sewage sludge by incineration. For example, U.S. Pat. No. 4,560,391 (issued to Ashworth) discloses a fuel composition comprised of non-dewatered sewage sludge and an organic fuel comprised of a fuel oil. U.S. Pat. No. 4,405,332 (issued to Rodriguez et al) discloses a fuel composition comprised of non-dewatered sewage sludge and a particulate solid fuel such as coal. U.S. Pat. No. 4,026,223 (issued to Robbins) discloses a sludge incinerator for use in the flash evaporation of water contained in high moisture sludges. The patent states at column 2, lines 31-33 that "waste oil or other flammable hydrocarbons may be introduced along with the sludge to aid in the ignition of the sludge." U.S. Pat. No. 4,145,188 (issued to Espenscheid et al) discloses a process for the liquefaction of municipal refuse and other solid organic wastes in a highly aromatic refinery petroleum solvent to provide a liquid fuel. U.S. Pat. Nos. 4,102,277 and 4,168,670 (each issued to Wall et al) disclose the incineration of lime-conditioned dewatered sewage sludge with a high sulfur fuel such as a fuel oil. British Pat. No. 1,198,958 discloses a solid fuel composition comprised of solid sewage waste, coal tip waste and waste oil. British Pat. No. 949,000 discloses the incineration of dewatered sewage sludge solids in admixture with a fuel oil.
U.S. Pat. No. 3,559,596 (issued to Ishii et al) is directed to a method and apparatus for the incineration of sewage sludge wherein a sludge is subjected to pressure and heated concurrently and then jetted into a combustion chamber. Water contained in the sludge is evaporated instantaneously with the jetting, with the remaining solids being incinerated. The patent discloses at column 2, lines 62-65 that the sludge-feeding duct is connected to a heavy oil-feeding duct. Example 1 also discloses the admixture of heavy oil with a sludge which is comprised of 80 percent by weight of water. The patent appears to be directed to the use of a partially-dewatered sludge as noted in the Examples (water content of 75-80 percent) and column 1, lines 15-20 wherein the partial dewatering of activated and digested sludge is discussed.
Japanese Patent Publication No. 55-94996 discloses a slurry fuel which includes particulate coal, oil, sewage sludge and a viscosity-lowering additive. The patent states that the sludge may contain from 30 to 90 percent water and generally envisions the use of partially dewatered sludge containing less than 50 percent by weight of water.
While the combining of sewage sludge with a fuel oil enables various advantages to be achieved, it has been found that the use of such a composition is somewhat less than practical under all circumstances. It has been discovered that the growth of microorganisms inherently present in the sewage sludge is substantially enhanced by the additional presence of the fuel oil. That is, the rate of growth of the microorganisms within a sewage sludge-fuel oil composition greatly exceeds the rate of growth of the microorganisms in either the sewage sludge or fuel oil alone.
While enhanced growth of such microorganisms would not generally be cause for concern when the fuel composition is combusted shortly after formation, fuel compositions must possess substantial long-term stability due to the fact that they are frequently stored prior to being combusted. It has been found that even the short-term storage of a sewage sludge-fuel oil composition under ambient conditions may result in the production of obnoxious by-products of microorganism growth which render contact with or exposure to the fuel composition highly undesirable if not impossible. Such undesirable states can be reached in as few as 1 to 3 days. Use of the fuel composition as a fuel once such an obnoxious state has been achieved is virtually precluded.
The prior art has recognized the inherent problems which exist with regard to microbial growth in both sewage sludge and hydrocarbon fuels and various methods of addressing such growth have been proposed.
For instance, U.S. Pat. Nos. 2,680,058 (issued to Harris) and 4,462,820 (issued to Grade et al) each teach the addition of a biocide to fuel oils to reduce microbial growth.
U.S. Pat. No. 4,108,771 (issued to Weiss); U.S. Pat. No. 4,541,986 (issued to Schwab et al); U.S. Pat. No. 3,883,303 (issued to Roberts); U.S. Pat. No. 4,081,366 (issued to O'Connell); U.S. Pat. No. 3,655,395 (issued to karnemaat); U.S. Pat. No. 4,462,820 (issued to Grade et al) and U.S. Pat. No. 4,479,820 (issued to Merk et al) teach the addition of biocides to aqueous waste compositions.
U.S. Pat. Nos. 3,642,135 (issued to Borden) and 3,296,122 (issued to karassik et al) each teach the heat treatment of sewage sludge during the disposal of same.
However, the prior art has not addressed the problem of microbial growth in fuel compositions comprised of sewage sludge and a hydrocarbon fuel oil, a composition which has been found to particularly foster microbial growth. Conventional sewage sludge incineration techniques also have been found to be inadequate to enable substantially complete combustion of the sludge solids to occur in such prior art compositions.